1. Field of the Invention
The present invention relates to the field of proton-exchange membrane fuel cells (PEMFCs).
It provides a solution enabling to limit the production of H2O2 during the use of bimetal PtxMy-type catalysts in the cathodes of PEM-type fuel cells, and this due to the forming of a nanostructured catalyst according to the invention.
2. Description of Related Art
PEMFCs are current generators having their operating principle, illustrated in FIG. 1, based on the conversion of chemical energy into electrical energy, by catalytic reaction of hydrogen and oxygen.
Membrane-electrode assemblies or MEAs 1, commonly called cell cores, form the basic elements of PEMFCs. They are formed of two electrodes (anode and cathode, respectively) separated by a polymer membrane 2, said membrane 2 being in direct contact with the catalytic layer (3, 4, respectively) present on each electrode. Thus, membrane 2 enables to separate anode and cathode compartments 5 and 6.
Catalytic layers 3, 4 are generally formed of platinum (Pt) nanoparticles supported by carbon clusters. Gas diffusion layers 7, 8 (carbon fabric, felt . . . ) are arranged on either side of MEA 1 to ensure the electric conduction, the homogeneous reactive gas distribution, and the discharge of products. A system of channels 9, 10 placed on each side of the MEA transport the reactive gases and discharge to the outside the excess water and gases.
At anode 3, the oxidation of hydrogen on the catalyst generates protons H+ and electrons e−. The protons then cross polymer membrane 2 before reacting with oxygen at cathode 4. The reaction of the protons with oxygen at the cathode (ORR, for “Oxygen Reduction Reaction”) causes the forming of water, of a little hydrogen peroxide (H2O2), and the production of heat. Improving the lifetime and decreasing the costs of PEMFCs are a major issue for the use and the development of cells for the consumer market. Therefore, identifying and understanding cell core aging phenomena are now essential.
PtxMy-type nanoparticles, M being a transition metal element (for example, Ni, Fe, Co, Cr), are alternative catalysts advantageous for the ORR reaction occurring at the cathode of PEMFCs (Stamenkovic B. et al., SCIENCE, 315, 2007, 493; Stamenkovic B. et al., J. AM. CHEM. SOC., 128, 2006, 8813-8819). Such bimetal catalysts, associating a second metal, less expensive than platinum, also enable to decrease the general cost of the catalyst. However, they have the specificity of increasing the production of hydrogen peroxide (H2O2), a by-product of the reaction with water, during the operation of PEMFCs.
Now, hydrogen peroxide is one of the chemical agents responsible for the degradation of membrane 2 by a method called Fenton reaction. It results from the dissociation of hydrogen peroxide into a radical and an ion, catalyzed by metal ions Fe2+ or Fe3+ generated by the corrosion of the bipolar plates:H2O2+Fe2+→Fe3++HO.+OH−H2O2+Fe3+→Fe2++HOO.+H+HOO.+Fe3+→Fe2++O2+H+H2O2+HO.→H2O+HOO.HO.+Fe2+→Fe3++OH−
The oxygen radical formed during the decomposition etches the proton-conducting polymer, thus degrading membrane 2 via complex chemical reactions (Romain Coulon et al. ECS Trans. 25 (35), 2010, 259-273).